When a program runs in CPU, it turns into a process. In what kind of data structure is the process stored in a system memory?
In Heap or Tree or some other data structures?
A process is not a single data structure. In the Windows operating system, for example, a running process has a number of different structures in its memory including a copy of its PE header and its environment variables. Here is a table from a debugger showing the layout of one process after it has been loaded into memory:
The various pieces of data have different structures. For example, the PE header is a record, but the heap is usually some kind of tree or list.